This invention relates in general to attenuator bypass valves used in combination with pulsation attenuators that reduce the amplitude of pressure fluctuations in hydraulic fluid lines of vehicular braking systems. In particular, this invention relates to an improved structure for such an attenuator bypass valve that includes a flow based orifice bypass function during high pump flow events while maintaining the orifice restriction and low noise, vibration, and harshness (NVH) behavior during low pump flow events.
Devices for autonomously generating brake pressure include pumps, such as piston pumps. In particular, the conveyance of brake fluid through piston pumps generates pulsations, which can spread audibly via brake circuits and also affect the noise level in the vehicle's interior. To dampen noise or pulsations, devices for autonomously generating brake pressure are known that feature an attenuator or an orifice on the outlet side of the pump.
The use of attenuators, which reduce amplitude of pressure fluctuations in hydraulic fluid lines of vehicular braking systems, is well known. In particular, attenuators are common in vehicular anti-lock braking systems (ABS) at the outlet end of an ABS hydraulic pump used to evacuate a low pressure accumulator. A hydraulic control unit (HCU) includes a housing having bores for mounting valves and the like and channels for directing fluid. An attenuator may be mounted in a bore in the HCU to significantly reduce the amplitude of high energy pressure pulses in the brake fluid at the outlet of the pump. These pressure pulses can create undesirable noise, which is transmitted to the master cylinder or its connection to the vehicle. These pressure pulses can also cause undesirable brake pedal vibrations.
A typical attenuator includes a chamber filled with brake fluid. An inlet passage delivers fluid from the outlet end of the pump to the chamber, and an orifice of substantially reduced diameter directs fluid from the chamber to an outlet passage. The restriction of fluid flow through the orifice attenuates pressure fluctuations as a result of the compressibility of the brake fluid. Thus, brake fluid in the chamber absorbs high energy fluid pulses and slowly releases the fluid through the orifice.
U.S. Pat. No. 8,671,680 shows an attenuator 7 in an HCU of a brake system. The attenuator 7 includes an attenuation chamber 8 having a fixed orifice 9 and a switchable orifice 10. The fixed orifice 9 is about twice as large as the switchable orifice 10. The switching function of the switchable orifice 10 is performed by a ball-check valve 11. The ball-check valve 11 is controlled by differential pressure and is configured to open at a predetermined cracking pressure. If the pressure difference at the ball-check valve 11 is not sufficient to open the ball-check valve 11, then fluid will flow initially through the switchable orifice 10, then through the fixed orifice 9 with the relatively larger orifice opening. When the pressure difference on the ball-check valve 11 reaches the predetermined cracking pressure, the ball 13 will lift up from its valve seat 14 so that the pulsating flow rate/volumetric flow moves directly from the attenuation chamber 8 through the orifice 9 with a large orifice opening. The ball-check valve 11 prevents fluid flow back through the orifice 9 to the attenuation chamber 8. Additionally, the ball 13 of the ball-check valve 11 operates in one of two positions: (1) a closed position when the pressure difference at the ball-check valve 11 is not sufficient to move the ball 13 against the force of the spring, and (2) a fully open position when the pressure difference on the ball-check valve 11 reaches the predetermined cracking pressure, and the ball 13 is lifted up from its valve seat 14 to allow fluid to flow through the ball-check valve 11.
U.S. Pat. No. 5,031,969 for example, discloses a throttle switchover valve 1 disposed in the brake circuit between a damper chamber 7 and the master cylinder 9. The throttle switchover valve 1 includes a throttle assembly 34 and a bypass valve 11. Fluid may flow either through the throttle assembly 34 or the bypass valve 11 and is therefore not variable.
U.S. Patent Publication No. 2013/0062933 A1 discloses an attenuator assembly 44 located in an attenuator chamber 102 of a housing 100 in a vehicle braking system 10 and includes an orifice 120 that defines a fluid dampening flow path. The orifice 120 has an outlet opening 120B. A biasing member 132 defines a closing member of the orifice 120. The size of the outlet opening 120B changes continuously between a first open position and a second open position.
It is desired however, to provide a brake system wherein fast brake pressure apply rates may be achieved using a piston type pump with pulsating output flow. It is further desirable to provide an improved structure for an attenuator bypass valve that includes a flow based orifice bypass function during high pump flow events while maintaining the orifice restriction and low NVH behavior during low pump flow events.